A new, non-invasive diagnostic blood test - polymerase chain reaction - for people at risk of an invasive mould infection (aspergillosis)

Review question. We planned to review the evidence about the accuracy of polymerase chain reaction (PCR) tests for diagnosing invasive aspergillosis (IA) in people with defective immune systems from medical treatment such as chemotherapy or following organ or bone marrow transplant.

Background: IA is a fungal disease caused by Aspergillus, a widespread mould. Most people breathe in Aspergillus spores every day without becoming ill, however people with weakened immune systems or lung diseases are at a higher risk of developing health problems due to Aspergillus. IA causes patient afflictions that are classically defined as invasive, saprophytic or allergic. Some types of IA are mild, but some of them are very serious. IA is the most common life-threatening, opportunistic, invasive fungal infection in people whose immune systems are compromised. Without treatment, most people with IA will die as a direct result, so early diagnosis and prompt administration of appropriate antifungal treatment are both critical factors to the survival of these people. As obtaining lung tissue is hampered by the risks involved, there is a need for new, non-invasive methods such as PCR to detect fungal nucleic acids in blood.

Study characteristics. The most recent search for studies was conducted in June 2015. Eighteen clinical studies reporting the evaluation of PCR tests prospectively in cohorts of people at high risk of IA were selected.

Study funding sources. None of the companies involved in the diagnosis of invasive fungal diseases funded any of the studies included in the review.

Quality of the evidence. Most studies were at low risk of bias and low concern regarding applicability. However, differences in the reference standard may have contributed to differences we found in the distribution of cases as being classified as IA or not.

Key results. Several PCR techniques were used in the studies. Pooling the data from the studies showed that sensitivity and specificity of PCR for the diagnosis of IA varied (from 58 to 80.5 % and from 78.5 to 95.2 %, respectively) according to the interpretative criteria used to define a test as positive. When used as a diagnostic criterion for IA in a population of 100 people with a disease prevalence of 13.0% (overall mean prevalence), a single PCR positive test would have missed three people with the disease, and falsely classified 19 people as having the disease who would be treated unnecessarily or referred for further tests. A requirement of two positive tests as a diagnostic criterion in a population with the same disease prevalence would miss six people with the disease and falsely classify three people as having the disease. These numbers should, however, be interpreted with caution because of the limitations of the reference standard in allowing consistent assessment of cases as being IA or not. Overall, PCR shows moderate diagnostic accuracy when used as a screening test for IA in high-risk patient groups. Importantly the sensitivity of the tests confers, with the low prevalence of the disease, a high negative predictive value such that a negative test allows the diagnosis to be excluded.

Authors' conclusions: 

PCR shows moderate diagnostic accuracy when used as screening tests for IA in high-risk patient groups. Importantly the sensitivity of the test confers a high negative predictive value (NPV) such that a negative test allows the diagnosis to be excluded. Consecutive positives show good specificity in diagnosis of IA and could be used to trigger radiological and other investigations or for pre-emptive therapy in the absence of specific radiological signs when the clinical suspicion of infection is high. When a single PCR positive test is used as diagnostic criterion for IA in a population of 100 people with a disease prevalence of 13.0% (overall mean prevalence), three people with IA would be missed (sensitivity 80.5%, 19.5% false negatives), and 19 people would be unnecessarily treated or referred for further tests (specificity of 78.5%, 21.5% false positives). If we use the two positive test requirement in a population with the same disease prevalence, it would mean that six IA people would be missed (sensitivity 58.0%, 42.1% false negatives) and three people would be unnecessarily treated or referred for further tests (specificity of 96.2%, 3.8% false positives). Galactomannan and PCR have good NPV for excluding disease but the low prevalence of disease limits the ability to rule in a diagnosis. The biomarkers are detecting different aspects of disease and the combination of both together is likely to be more useful.

Read the full abstract...
Background: 

Invasive aspergillosis (IA) is the most common life-threatening opportunistic invasive mould infection in immunocompromised people. Early diagnosis of IA and prompt administration of appropriate antifungal treatment are critical to the survival of people with IA. Antifungal drugs can be given as prophylaxis or empirical therapy, instigated on the basis of a diagnostic strategy (the pre-emptive approach) or for treating established disease. Consequently there is an urgent need for research into both new diagnostic tools and drug treatment strategies. Newer methods such as polymerase chain reaction (PCR) to detect fungal nucleic acids are increasingly being investigated.

Objectives: 

To provide an overall summary of the diagnostic accuracy of PCR-based tests on blood specimens for the diagnosis of IA in immunocompromised people.

Search strategy: 

We searched MEDLINE (1946 to June 2015) and EMBASE (1980 to June 2015). We also searched LILACS, DARE, Health Technology Assessment, Web of Science and Scopus to June 2015. We checked the reference lists of all the studies identified by the above methods and contacted relevant authors and researchers in the field.

Selection criteria: 

We included studies that: i) compared the results of blood PCR tests with the reference standard published by the European Organisation for Research and Treatment of Cancer/Mycoses Study Group (EORTC/MSG); ii) reported data on false-positive, true-positive, false-negative and true-negative results of the diagnostic tests under investigation separately; and iii) evaluated the test(s) prospectively in cohorts of people from a relevant clinical population, defined as a group of individuals at high risk for invasive aspergillosis. Case-control studies were excluded from the analysis.

Data collection and analysis: 

Authors independently assessed quality and extracted data. For PCR assays, we evaluated the requirement for either one or two consecutive samples to be positive for diagnostic accuracy. We investigated heterogeneity by subgroup analyses. We plotted estimates of sensitivity and specificity from each study in receiver operating characteristics (ROC) space and constructed forest plots for visual examination of variation in test accuracy. We performed meta-analyses using the bivariate model to produce summary estimates of sensitivity and specificity.

Main results: 

Eighteen primary studies, corresponding to 19 cohorts and 22 data sets, published between 2000 and 2013 were included in the meta-analyses, with a median prevalence of IA (proven or probable) of 12.0% (range 2.5 to 30.8 %). The majority of people had received chemotherapy for a haematological malignancy or had undergone a hematopoietic stem cell transplant. Several PCR techniques were used among the included studies. The sensitivity and specificity of PCR for the diagnosis of IA varied according to the interpretative criteria used to define a test as positive. The mean sensitivity and specificity were 80.5% (95% CI; 73.0 to 86.3) and 78.5% (67.8 to 86.4) for a single positive test result, and 58.0% (36.5 to 76.8) and 96.2% (89.6 to 98.6) for two consecutive positive test results.

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